At present a final evaluation of the benefit of therapeutic drug monitoring (TDM) of psychotropic drugs is not possible. This is a result of among other things the deficit of controlled studies which directly? compare fr treatment with and without TDM. On the other hand the level of evidence of TDM of psychotropic drugs, according to the criteria evidence-based medicine (EBM), is far above the level of basic research and case reports. The scientific and methodical basis of TDM was established over the last three decades in extensive clinical and chemical analytical research which investigated the relationships between serum concentrations and clinical effect as well as side-effects and intoxications of a number of drugs in various clinical situations. Candidates for a routine TDM are, apart from the already established lithium the high-potency neuroleptics haloperidol, fluphenazine, trifluoperazine and perphenazine the atypical neuroleptic clozapine as well as the tricyclic antidepressants depressants imipramine, desipramine, amitriptyline and nortriptyline. However, an individual and balanced application of TDM is also recommended for many other high-potency neuroleptics, chlorpromazine and tri- and tetracyclic antidepressants. There is need for more research on the new substances of the group of SSRIs and atypical neuroleptics. The degree of the application of the TDM of psychotropic drugs is considerably lower than of anticonvulsive drugs, aminoglycoside antibiotics and cardioactive drugs. Two reasons for this under-development of the TDM of psychotropic drugs include gaps in clinical research, as mentioned above, but also an insufficient interpretation of existing results and open questions of cost effectiveness Moreover; whereas TDM is used to avoid intoxications with anticonvulsive, anti-biotic and cardioactive drugs, the chance is neglected to avoid mild and latent CNS-toxicity of neuroleptics and tricyclic antidepressants by using TDM As a preliminary conclusion, an optimisation of drug therapy seems possible by, means of the assay of serum concentrations of psychotropic drugs in a TDM programme in psychiatric clinics. It may be expected that the method can be established as a useful completion of the dose-guided and clinically guided treatment.